Phonocardiograms from patients with Starr-Edwards aortic prostheses, series 1000 and 1200, have been analyzed with the aid of a hybrid computer system. The following parameters are obtained from the phonocardiogram for evaluation: 1) a correlation coefficient (RXY) for the opening prosthetic aortic valve sound, 2) a prosthetic aortic opening sound intensity to closing sound intensity ratio (AO/AC), and 3) a prosthetic aortic opening sound energy to closing sound energy ratio within a high frequency window (Hi-Freq AO/AC). A spectrographic analysis was performed on a representative sample of our patient population to evaluate the use of this technique in the detection of silastic ball variance.